As stated above, a wide variety of components are used in end products. Higher strength has further been required in starting materials of those components on order to prevent deterioration of rigidity due to recent progress in size reduction and weight reduction (reduction in plate thickness or reduction in cross sectional area) of products. Size reduction and weight reduction of products and components result in efficient utilization of precious resources, and also contribute to environmental problems solution. Meanwhile, complication and high preciseness in shapes of components have been pursued, and thus starting materials having excellent formability is also required.
In such requirements, high strengthening in conventional metallic materials inevitably results deterioration of formability, and high strength is inconsistent with excellent formability. A spring is subject to repetitive deformation, and often undergoes fatigue failure at an early stage due to local concentration of stress. Hence, a need for materials for spring members having high strength and excellent formability and fatigue resistance increases.
In general, one of biggest feature of stainless steel is excellent corrosion resistance, and stainless steel has been used as a starting material for spring members. Specifically, metastable austenitic stainless steel such as SUS301 and SUS304 has been commonly used as a starting material for spring members. This is because, in the metastable austenitic stainless steel, transformation from an austenitic parent phase into a rigid martensitic phase (deformation-induced martensitic transformation) is caused by cold working, so that high strength can be relatively easily obtained, and the strength can be adjusted in a wide range.
The metastable austenitic stainless steel is excellent in formability because its austenitic parent phase exhibits high elongation, and deformed regions become hardened through transformation into the martensitic phase, as stated above, and soft non-deformed regions are more likely to be deformed, by which the entire material becomes uniformly deformed (TRIP effects), so that excellent formability is exhibited as well. Because of the above features, the metastable austenitic stainless steel is classified as a stainless steel strip for a spring in JIS Standard (JIS-G-4313), and its mechanical property is also specified by JIS Standard.
A degree of work hardening exhibited by the metastable austenitic stainless steel changes with various factors, which often makes it difficult to stably obtain desired properties with a desired plate thickness of a product. It is also a problem that load charged especially at the time of rolling increases because of demands of thinner plate and higher strength for addressing the current needs of size reduction and weight reduction of spring members. Further, the metastable austenitic stainless steel contains a plenty of Ni that is an expensive and rear alloy element and hence is expensive.
Meanwhile, martensitic stainless steel, such as SUS403, SUS 410, and SUS420, in which high strength is obtained through transformation into a hard martensitic phase as an intermediate phase by heat treatment (quenching), is employed as a starting material for spring members. In addition, martensitic stainless steel is often used as a starting material for utilizing a mixed phase structure in combination with a ferrite phase. These stainless steels hardly contain Ni; therefore, it is more inexpensive than the above metastable austenitic stainless steel.
As the above-stated martensitic stainless steel, Patent Document 1 discloses a high strength stainless steel with a composite phase structure; Patent Document 2 discloses a high strength dual phase stainless steel strip or sheet; Patent Document 3 discloses a high strength double phase stainless steel strip for a steel belt; Patent Document 4 discloses a mixed phase stainless steel for a gasket; Patent Document 5 discloses a high strength duplex stainless steel sheet having high elasticity, and Patent Document 6 discloses a high-strength stainless steel plate having excellent ductility.